In such vehicles, the installation of the rail truck is accomplished by raising one end of the chassis, and then, after having disengaged the center pin socket of the carriage, lowering the chassis and its suspension until a hemispherical center-bearing counter-element of the semi-trailer is inserted in the center pin socket of a rail truck.
This results in some difficulty in centering the center-bearing counter-element member of the semi-trailer because of the low tolerances inherent in such operations, and necessitates that the pneumatic suspension of the highway carriage have high capacity to enable the fully loaded semi-trailer to be raised by the pneumatic suspension. This occasions a costly type of suspension. The reason is the need to execute an extra excursion of about 100 millimeters in order to disengage the center pin socket from the center-bearing count-element which is affixed to the car chassis. Additionally, there is an increase in the lifting time.
In order to remove these disadvantages for semi-trailers not having a removable highway carriage, builders of bimodal (road and rail) vehicles have devised apparatus which enable rail trucks to be used without the need for raising the end of the semi-trailer which is disposed above the center pin socket of the carriage. This apparatus has disadvantages, however, that interferes with the operation of railroad-type coupling devices between the semi-trailers. Furthermore, the apparatus may be quite complex, and does not permit an elastic coupling.
Also, the semi-trailers according to the above-cited Italian patent application, whether of the type adapted to travel on rails without a highway carriage, or the type for which rail use requires that the highway carriage be held elevated, further require, in order to be connected into a normal railroad train having some normal cars, that specially adapted rail trucks be supplied (hereinafter, "shield trucks") to be used on both the rear and front parts of the attached sequence of semi-trailers, and that the rail trucks can be rapidly mounted and will not require excessive maintenance.